If you are naturally carbonating in the keg at room temp, you need to add about 1/3 to1/2 the amount you normally would add to bottles. So about 2-2.5oz of priming sugar to get the typical 2.2-2.5 atmospheres you would for a typical ale, more or less for highly carbonated or lower carbonated styles.
Boil to dissolve and sanatize the priming sugar just like you would for bottles. Cool it down to about 70-80F, then add it to the bottom of the corny keg and rack your beer on top.
You then still need to seat your corny keg top with a blast of 30PSI CO2 from your tank, purge the small bit of oxygen, check for leaks, and if all's good, throw the keg into an interior closet or anywhere with a stable temp as close to 70F as possible, carb and condition for about 3 weeks for an average OG beer (over about 1.05-1.06 takes proportionally longer). At that point, it's just like a bottle.
Good luck!